(1) Field of the Invention
The present invention relates to a mesoporous catalyst having a large specific surface area and a monolith catalyst comprising the mesoporous catalyst applied to gas channel inner walls of a monolith formed body. The use of the monolith catalyst of the present invention allows highly efficient purification treatment of NOx contained in lean burn vehicle exhaust gases.
(2) Description of the Related Art
The leading conventional three way catalyst (i.e., a catalyst containing three different metal atoms) for vehicle exhaust gas purification has a monolith body with cordierite as the catalyst support having gas channel inner walls. This three way catalyst further has a structure in which active alumina particles (with a size of several micrometers to several tens of micrometers) contain platinum-palladium-rhodium particles (with a size of several hundred nanometers to several micrometers) as the catalyst which are applied to the gas channel inner walls of the monolith body. The active alumina particles are an aggregate of microparticles with a size of several tens of nanometers to several hundred nanometers, and adsorbs the catalyst particles in the interstices between the microparticles. The interstice-type pore has a small spatial expanse (namely, planar), and is basically different from the through-hole pore structure (hereinafter referred also to as a pore channel) wherein the pore expands networkwise and is present in the synthetic zeolite and the mesoporous material used in the present invention. That is, the conventional catalyst particles do not have a structure wherein the catalyst particles are captured in three-dimensional pores.
The catalyst supported on a molecular sieve-like synthetic zeolite is generally referred to as a pore-supported catalyst, and then for distinguishing from this, conventional three way catalysts are, hereinafter, described as adsorption-supported catalysts. The three way catalyst is very effective for the exhaust gas treatment of gasoline vehicles, but is scarcely effective for the exhaust gas treatment of diesel vehicles running with gas oil fuel. In particular, the catalyst development to purify NOx emission at 150 to 200° C., which is emitted during transit is an unsolved problem also in the field of catalyst chemistry. Even at present, no practical catalyst for the diesel vehicle exhaust gas treatment is known. The major reason for this is the remarkable decrease in the activity of the above described three way catalysts in a relatively high concentration oxygen atmosphere of the diesel vehicle exhaust gas. Although the oxygen concentration in the gasoline vehicle exhaust gas is 1% or less, the oxygen concentration in the diesel exhaust gas is usually 5% or more because the air/fuel ratio of gas oil is more than several times that of gasoline. For gasoline vehicles, since the coexisting oxygen is controlled at 1% or less by burning at near the stoichiometric air/fuel ratio showing a stoichiometric mixing weight ratio of air to fuel, the combustion is referred to as rich burn; by contrast, in the diesel fuel combustion, since the intake air flow is largely in excess of the theoretical one, and the fuel supply amount is relatively small, the combustion is referred to as lean burn. This is because the three way catalyst is almost deactivated when the oxygen concentration becomes 5% under this combustion condition.
Generally, industrial catalysts are often used in the state of being supported on porous materials. The pores of porous materials are, according to IUPAC, classified into micropores of 2 nm or less in pore diameter, mesopores of 2 to 50 nm and macropores of 50 nm or more. No single porous material having a wide distribution ranging from micro to meso is known other than active carbon. Recently, silica, alumina and silica-alumina mesoporous molecular sieves have been developed which have the pore peak at a position of a few nanometers and a very large specific surface area of 400 to 1,100 m2/g. The molecular sieves are disclosed, for example, in JP-A-5-254827, JP-A-5-503499 and JP-A-6-509374.